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Author Topic: Basic L/R VHF Parasitic Oscillation Suppressor Design  (Read 71075 times)
AG6K
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« on: October 02, 2011, 05:19:09 AM »

Goal:  reduce the amplification of a HF / MF amplifier at the unavoidable VHF parasitic resonance** in the anode circuit by lowering the VHF-Q of the L/R suppressor - which in turn lowers the VHF RL at the anode, thereby reducing VHF amplification and the possibility of VHF oscillation.
** 43MHz to 155 MHz in amplifiers I have encountered.
  To lower the Q  of a L/R parallel circuit one needs to increase R.  The obvious way to do this is to increase the resistance of R.  However as resistance increases, so does dissipation - and that problem is most severe at 28MHz since that is the band where the V-drop across L is max.  Since the R component of a suppressor needs to have low self-inductance to function properly, suitable resistors are few.  A workaround is to use resistance-wire for L.  Even though its resistance is in series, by doing a series to parallel equivalent conversion one can see that it has the same effect that a parallel resistance has.  This reduces the dissipative burden on R by shifting part of it to the resistance-wire in L.  Some builders of resistance wire suppressors report seeing the L glow red during A0/N0N testing at 28MHz. 
- note 1 - see: "Calculating Power Dissipation in Parasitic-Suppressor Resistors", March, 1989 QST Magazine.
- note 2 - for optimal results, the reactance of the L at the parasitic resonance needs to have c. the same # of ohms that the R has.   
-  end
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G3RZP
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« Reply #1 on: October 02, 2011, 06:38:27 AM »

> To lower the Q  of a L/R parallel circuit one needs to increase R. <

???NO NO NO.

Q = XL/R in a series L-R circuit.

If increasing R lowered Q, then a high Q circuit would want a short circuit across it! 

For a parallel circuit, Q = R/X

Thus Q is proportional to R, and to lower Q, you DECREASE R
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AG6K
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« Reply #2 on: October 02, 2011, 01:48:36 PM »

> To lower the Q  of a L/R parallel circuit one needs to increase R. <

???NO NO NO.

  "- note 2 - for optimal results, the reactance of the L at the parasitic resonance needs to have c. the same # of ohms that the R has"
  •  therefore when R is increased, XL is increased  a similar # of ohms. 

Q = XL/R in a series L-R circuit.

If increasing R lowered Q, then a high Q circuit would want a short circuit across it! 

  So you are saying Q would be less with more R across the paralleled L ? 


For a parallel circuit, Q = R/X

Thus Q is proportional to R, and to lower Q, you DECREASE R

   to lower Q in a L/R parallel circuit, a 0-ohm resistor would yield the lowest Q ?
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W8JI
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« Reply #3 on: October 02, 2011, 02:12:48 PM »

   to lower Q in a L/R parallel circuit, a 0-ohm resistor would yield the lowest Q ?

Yes, a short would have the lowest Q. G3RZP was correct, and AG6K was opposite of correct.

This is why nichrome is such a generally bad idea for suppressors unless we want low HF Q and the same VHF Q. The primary low frequency path is through the inductor. The primary high frequency path is through the capacitor. If we make the inductor part of the system lossier, we decrease Q most on the lowest frequency.

Imagine a parallel tuned tank circuit Rich, with an open circuit across it. It has highest Q with the highest resistance loading it.
As we decrease resistance across the tank, Q decreases.

73 Tom
« Last Edit: October 02, 2011, 02:31:26 PM by W8JI » Logged
AG6K
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« Reply #4 on: October 03, 2011, 06:29:25 AM »

   to lower Q in a L/R parallel circuit, a 0-ohm resistor would yield the lowest Q ?

Yes, a short would have the lowest Q. G3RZP was correct, and AG6K was opposite of correct.
  So the way to lower the Q of an L/R VHF parasite suppressor is to short it out?
[/quote]
This is why nichrome is such a generally bad idea for suppressors unless we want low HF Q and the same VHF Q. The primary low frequency path is through the inductor. The primary high frequency path is through the capacitor. If we make the inductor part of the system lossier, we decrease Q most on the lowest frequency.

Imagine a parallel tuned tank circuit Rich, with an open circuit across it. It has highest Q with the highest resistance loading it.
As we decrease resistance across the tank, Q decreases.

73 Tom
[/quote]
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G3RZP
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« Reply #5 on: October 03, 2011, 06:43:42 AM »

The math is very easy - even for someone as bad at math as I am. Use the series - parallel transform of

 Xs = (Rp squared  times Xp)/ (Rp squared + Xp squared)

and Rs = (Xp squared times Rp)/(Rp squared + Xp squared)

Let Q for the series case be Xs/Rs.

Substitute the equivalents from above, a whole lot of cancellation happens, and you end up with Q = Rp/Xp = Xs/Rs

It is unfortunate that you can't do correct mathematical formula here on eHam.net.
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AG6K
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« Reply #6 on: October 03, 2011, 09:31:45 AM »

The math is very easy - even for someone as bad at math as I am. Use the series - parallel transform of

 Xs = (Rp squared  times Xp)/ (Rp squared + Xp squared)

and Rs = (Xp squared times Rp)/(Rp squared + Xp squared)

Let Q for the series case be Xs/Rs.

Substitute the equivalents from above, a whole lot of cancellation happens, and you end up with Q = Rp/Xp = Xs/Rs

It is unfortunate that you can't do correct mathematical formula here on eHam.net.
We could if we could attatch JPEGs.
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N4NYY
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« Reply #7 on: October 03, 2011, 09:35:06 AM »

Quote
We could if we could attatch JPEGs.

You don't have to. Use a photo server and post the links. That is what I do.
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AG6K
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« Reply #8 on: October 03, 2011, 10:22:37 AM »

Quote
We could if we could attatch JPEGs.

You don't have to. Use a photo server and post the links. That is what I do.

  good idea.  tnx
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N4NYY
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« Reply #9 on: October 03, 2011, 10:39:25 AM »

Rich,

I use photobucket. I use it for ebay, eham, etc. I mostly use it for information gathering as I am one of the people that asks far more questions that I answer. And you are getting the hang of the quote feature, which makes the thread easier to follow.
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AG6K
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« Reply #10 on: October 03, 2011, 10:59:13 AM »

Rich,

I use photobucket. I use it for ebay, eham, etc. I mostly use it for information gathering as I am one of the people that asks far more questions that I answer. And you are getting the hang of the quote feature, which makes the thread easier to follow.

 It's not easy to teach us old dogs new tricks.  This is the first discussion forum I have participated in that does not auto indicate levels.. . .  ¿ What character do u see at the beginning of my replies ?   tnx
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N4NYY
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« Reply #11 on: October 03, 2011, 11:10:28 AM »

Quote
 It's not easy to teach us old dogs new tricks.  This is the first discussion forum I have participated in that does not auto indicate levels.. . .  ¿ What character do u see at the beginning of my replies ?   tnx

That one
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WB6BYU
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« Reply #12 on: October 03, 2011, 02:25:18 PM »

A little square box that says

F8
FF

I see it on both the Mac and the PC.


The software automatically handles nested quotes if you put yours at the end, but when you add them
in the middle you have to add the necessary number of {/quote} {quote} pairs around it.

The "preview" function is very useful for getting it right.
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K9FV
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Posts: 494




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« Reply #13 on: October 03, 2011, 07:07:27 PM »

To get the format to quote correctly try this - replacing "{" with "[" without the "" marks.

{quote author=WB6BYU link=topic=77880.msg543398#msg543398 date=1317677118} (this starts original quote)
A little square box that says
{/quote} (this ends first section of original quote)

This is where I might type more text., (this will appear in white space as my text)

{quote author=WB6BYU}  (this is required to start next quote section)
The "preview" function is very useful for getting it right.
{/quote} (this will end quote section)

more verbiage typed in, and I hope this helps anyone who is trying to get multi-quotes to format.

73 de Ken H>
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AG6K
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« Reply #14 on: October 03, 2011, 09:22:10 PM »

To get the format to quote correctly try this - replacing "{" with "[" without the "" marks.

{quote author=WB6BYU link=topic=77880.msg543398#msg543398 date=1317677118} (this starts original quote)
A little square box that says
{/quote} (this ends first section of original quote)

This is where I might type more text., (this will appear in white space as my text)

{quote author=WB6BYU}  (this is required to start next quote section)
The "preview" function is very useful for getting it right.
{/quote} (this will end quote section)

more verbiage typed in, and I hope this helps anyone who is trying to get multi-quotes to format.

73 de Ken H>

••  Thanks Ken.  Preview helps.  .  .  This application was not intended to use with Apple's Unix-based OS 10.4.  I find this semi amazing since Apple Computer is now the most valuable company in America.  cheers
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